The field of the invention relates to the measurement of capacitance, in particular the measurement of lossy capacitance. Stated another way, the measurement of capacitance shunted by a low resistance.
As the world's petroleum reserves become depleted, interest has increased in adapting vehicles to run on a mixture of gasoline and alcohol (including ethanol and methanol). With present technology, vehicles may be adapted to run on a gasoline/alcohol mixture by altering the engine's air/fuel ratio and/or ignition timing to compensate for the lower energy density of these fuel mixtures. Since the fuel mixture may vary depending upon both gasoline and alcohol availability in a specific locality, it is desirable to have an on-board sensor for providing an engine controller with an indication of gasoline/alcohol mixture in the fuel tank.
Gasoline and alcohol have substantially different dielectric constants. Accordingly, measurement of the capacitance between two parallel plates suspended in the fuel tank may provide an approximation of the gasoline/alcohol mixture. However, the gasoline/alcohol mixture contains contaminants, such as sodium or calcium ions, from dessicants, thereby adding a resistive component between the plates. This parallel resistive component may be sufficiently low to overwhelm capacitive measurement unless very high frequencies, such as 10 MHz, are used. For example, when measured at 30 KHz, the resistive current component between the plates may be approximately 20 times greater than the capacitive current component.
A prior approach for on-board measurement is disclosed in IEEE Transactions on Vehicular Technology, Volume VT-27, No. 3, August 1978, page 142, entitled "An On-Board Sensor For Percent Alcohol", by J. W. Hille and P. R. Rabe. This approach requires a tuned circuit for comparing the capacitance to be measured to a calibrated reference. A disadvantage with this approach is that variations in the reference will adversely affect the measurement. Further, the problem of a high resistive current component does not appear to be addressed.
An approach to measurement of capacitance in general, rather than the capacitance of a gasoline/alcohol mixture, is disclosed in U.S. Pat. No. 3,947,760. A circuit is disclosed for measuring both the resistance and capacitance of a capacitor. The ratio of charging and discharging times of two integrative capacitors is compared to determine the desired measurements. This circuit has the disadvantage of requiring a variable voltage source. In addition, the problem of a high resistive component overwhelming the capacitive current component does not appear to be addressed.